The owner of the LGS I frequent has several ways to help deter straw buyers.
In additional to common signage about the penalties for straw purchasing if he gets a whiff of someone buying a gun for someone else he will flat out ask them and then say, "I've got to ask because a lot of my customers are law enforcement officers and if one walks through the door and thinks he is seeing a straw purchase we are going to have a problem".
Works like a charm. Some people find an excuse to back out or leave and not come back while others go ahead with the transaction but don't return and purchase more firearms. He does have a family member who is a LEO so I'm sure that factors into him trying to keep a few guns off the street over a few more dollars in his pocket.